It’s not blasphemous at all. The “co” refers to with Jesus, not with us. I think I have a good understanding of the Coredemption since I’ve done a fair amount of reading on it. Here’s what Fr Peter Damian M Fehlner, FFI says:In that sublime alliance of the hearts
Jesus and Mary, Redeemer and Coredemptrix, together merit de condigno the same work, our salvation, according to an order willed from all eternity.So Jesus and Mary together merit our salvation. He writes further:In conclusion: we may say that, in virtue of the divine salvific counsels ordaining a most perfect redemption, our Lady as Coredemptrix is included with Christ, the One Mediator.
Under him she merits de condigno all which he merits, except the grace to be the Immaculate Mother of God and Coredemptrix. This is precisely what preservative redemption in the full sense means in so far as it differentiates her cooperation with the Savior from that both of men and of angels: to be able to be actively engaged with the One Mediator in the historic work of salvation: as his Mother and as ours, both at its initiation and at its consummation, and therefore both before and after that consummation. She is therefore the link, or as St. Bernard put it, the neck, uniting Head and Body of the Church, only on the assumption that
she alone is the Coredemptrix, not only offering, but in some way part of the sacrifice of Christ.
Once we realize that the compassion of Mary, like her Son’s suffering, is that of a public person, then the title, “Sorrowful Mother,” and its universal acceptance in the faith and devotion of the Christian people, is revealed to be a synonym of Coredemptrix.
So Mary merits at the Cross everything that her Son merits except the grace to be the Immaculate Mother of God and to be Coredemprix. Mary, like her Son, is a public person and so her suffering is redemptive. And she not only offers her Son’s suffering but is in some way part of his sacrifice.
Here’s the full paper of Fr Peter:
voxpopuli.org/book_2_10.php
I should note that some theologians disagree with Fr Peter as to the nature of Mary’s merit. This is noted in Fr Peter’s paper.